1. Field of the Invention
The present invention relates generally to a method for localizing and imaging portions of a specimen having a defined time-dependent signal using a scanning microscope focused thereon and to an apparatus for implementing the present method.
2. Description of the Prior Art
Large scale integrated circuits can be function checked using a computer controlled test system. The test system identifies existing errors by analyzing the voltage levels at the outputs of the circuit depending on the bit patterns supplied to the circuit. Using such a method localizing of errors is done only with great difficulty. As such, additional measurements for the inside portions of large scale integrated circuits must be carried out, particularly during the development phase of the circuit's manufacture. Electron beam measuring methods have proven particularly suitable for such tests in all areas of development and manufacture of microelectronic components. The dynamic distribution of potential in the integrated circuits is imaged in methods known as "voltage coding", or "logic-state mapping", for example. Also, temporal curves of voltage at individual nodes on the circuit are defined in a method identified as "waveform measurement".
As disclosed in the publication H.-D. Brust "Frequency Tracing and Mapping: Novel Electron Beam Testing Methods" (Proceedings of the Microcircuit Engineering Conference, Berlin, September 1984, pp. 412-425), a method for localizing and imaging the interconnections of an integrated circuit carrying a specific frequency is known. However, it is not possible with the disclosed method to distinguish between signals of the same freqency but having different signal curves.
Therefore, it is proposed to provide a method whereby interconnections of an IC carrying a defined electrical signal are localized and imaged, as disclosed in German Patent Application No. P 34 28 965. The disclosed method is referred to as "logic-state tracing" (LST). Not only are the interconnections carrying the sought-after signal displayed brightly on a visual display means, but also, under certain conditions, regions of the specimen under test in which particularly high or low secondary electron currents occur are also displayed brightly. Such high or low secondary electron currents can occur due to material or topographical contrasts of the specimen under test.
U.S. Pat. Nos. 4,220,853 and 4,223, 220 disclose electron beam measuring instruments.